As disclosed in Non-Patent Documents 1 and 2 referred to hereinafter, in order to reduce a load on an engine to improve a mileage in a commercial car such as a truck, there has been recently proposed a hybrid system that uses the help of the motor at a time of starting and at a time of traveling in which a load is applied to an engine such as at a time of traveling on an uphill road.
FIG. 10 illustrates a power transmission mechanism for hybrid vehicle disclosed in Non-Patent Document 1, and as illustrated in the drawing, regarding an arrangement of driving systems of the power transmitting mechanism, an engine 1, a main clutch (single disk clutch) 3, a motor (motor/generator) 5, a transmission 7, a final reduction gear 9 are all disposed coaxially in this order. Further, a rotor 13 of the motor 5 is attached to an engine drive shaft 11 that couples between the main clutch 3 and the transmission 7, and a propeller shaft 15 is coupled between the transmission 7 and the final reduction gear 9.
Further, in the hybrid system, a vehicle starts to move using only a driving force of the motor 5 in a state of disengaging the main clutch 3, rotations of the engine 1 and the motor 5 are synchronized at a specified vehicle speed (around about 5 to 6 km/h), and after the main clutch 3 is engaged, the vehicle starts to travel with the use of only the engine 1.
Further, it is structured such that at a time of traveling in which a load is applied such as when traveling on an uphill road, high-power traveling in which the engine 1 and the motor 5 are used in a combined manner in accordance with a depressing amount of an accel pedal is started to be performed, and at a time of deceleration/braking, by disengaging the main clutch 3 in accordance with situations and making the motor 5 to be inversely operated to apply a regenerative brake equivalent to an engine brake, a braking energy (regenerative energy) is collected in a battery.
Further, FIG. 11 illustrates a power transmitting mechanism for hybrid vehicle disclosed in Non-Patent Document 2, and in the power transmitting mechanism, a motor output shaft 19 is structured separately from an engine drive shaft 11, and a clutch mechanism 21 formed of a dog clutch is attached between the motor output shaft 19 and the engine drive shaft 11.
Furthermore, as illustrated in FIG. 12, Patent Document 1 discloses a power transmitting mechanism for hybrid vehicle in which outputs of an engine 1 and a motor 5 are synthesized in a planetary gear mechanism 23 to be output to a transmission 7 side.
As illustrated in the drawing, this power transmitting mechanism is characterized in that it is structured such that a ring gear 25 of the planetary gear mechanism 23 and a motor output shaft 19 of the motor 5 are coupled and a sun gear 27 of the planetary gear mechanism 23 and an engine drive shaft 11 are coupled to output power from a carrier 29 of the planetary gear mechanism 23 to the transmission 7 side, and meanwhile, when the ring gear 25, the carrier 29, and the sun gear 27 of the planetary gear mechanism 23 have mutually substantially the same predetermined coupling rotation speed, the carrier 29 and the ring gear 25 are integrally coupled through centrifugal clutches 31 to make a transmission ratio to be 1.
Further, with the use of this power transmitting mechanism, when the engine 1 is driven to make the sun gear 27 rotate at a coupling rotation speed of the centrifugal clutches 31 when a vehicle is stopped, a regenerative braking is applied to the motor 5 which is in a state of being inversely rotated to smoothly stop the inverse rotation and operation of the motor is smoothly changed to a normal rotation powering, the vehicle starts to move smoothly, and when a rotation speed of the motor 5 is increased while controlling the driving of the engine 1 so that the sun gear 27 keeps rotating at the coupling rotation speed, a rotation speed of the ring gear 25 soon reaches the coupling rotation speed, and the carrier 27 and the ring gear 25 are integrally coupled through the centrifugal clutches 31.    Patent Document 1: Japanese Unexamined Patent Application Publication No. H09-14385    Non-Patent Document 1: “Monthly Automotive Engineering” October 2006 issue, published by TETSUDO-NIPPONSHA CO., LTD, pages 47-49.    Non-Patent Document 2: “Isuzu Technical Journal” No. 113, 2005, published by Isuzu Motors Limited, pages 66-69